1. Field of the Invention
The present invention relates to a circuit substrate, a circuit module and a method for manufacturing the circuit substrate, and more specifically, to a circuit substrate on which electronic components are to be mounted, a circuit module and a method for manufacturing the circuit substrate.
2. Description of the Related Art
For mounting an electronic component, such as an IC, on a circuit substrate, the following technique is generally applied. Flat electrode pads are arranged in an array manner on the main surface of a circuit substrate. On the main surface of an IC, solder bumps are formed in an array manner. The IC is placed on the circuit substrate in such a manner that the solder bumps are aligned with the respective electrode pads, and the solder bumps are reflowed. The solder bumps are melted to fix the electrode pads, and the IC is thus mounted on the circuit substrate.
However, the circuit substrate and the IC have different thermal expansion coefficients. Consequently, for example, the circuit substrate may be warped by heat, and, thus, this mounting technique may result in connection failure between the circuit substrate and the IC. Accordingly, Japanese Patent No. 3203731 and Japanese Unexamined Patent Application Publication No. 2005-45143 disclose a semiconductor device substrate on which columnar bumps, but not flat electrode pads, are formed so as to protrude from the main surface thereof.
The columnar bump has a larger height than the electrode pad and is accordingly elastic. Even if the circuit substrate is warped by heat, the columnar bump can absorb the warping. Thus, the semiconductor device substrates disclosed in Japanese Patent No. 3203731 and Japanese Unexamined Patent Application Publication No. 2005-45143 can prevent the occurrence of connection failure between the circuit substrate and the IC.
Such a semiconductor device substrate however increases the height of the circuit substrate by the height of the columnar bumps.